Plaster composition and process



" ya Eels. e, 944 I stone molds, and

amaze msrnn oomosmor: an success.

Mauve] O. Bailey,

Elmhurst, m, assig-nor to United States Gypsum company, ilhlcaco, 1

a corporation of ois (El. MIG- ild) This invention relates to 'afcomposition or matter and process of sum plastic composition and process for preparing it, which composition is' particularly suitable for use in forming molds where fidelity, accuracy of impression, and long mold life are desirable, such as for example in pottery molds, artificial the like.

In the past it has heenimpractical toproduce a pottery or industrial moldingplastcr ing a normal consistency below about 60 cubic centimeters (normal consistency being herein defined as the number of cubic centimeters of water which, when edwith 100 grams of plaster, will produce a slurry or such fluidity that it will just pour from a cup). Drdinary calcined sum which is prepared according to the prior art process in a kettle calciner or total? calciner at normal. atmospheric. temperatures a may be ground suficientlyflne to produce c having desired finish, smoothness, plasticity, freedom from pin holes, etc; However, when this type of gypsum is ground to this desired degree of fineness, the consistency of the product is above about so cubic centimeters. For many uses it is desirable that a casting plaster have a norgial consistency of less than oocubic centime 1'3.

meeting such a need for such a product.

The plasters which have been heretofore used for the preparation of casts and molds makingthe same. More par- Y ticularly, it relates to an improved calcined sypcated in the foregoing example except that the addition oi-hysroscopic salt: is omitted and the calcination is carried out at a-hisher temperaturc-or for a longer period of time to permit the formation 01' a substantial amount of soluble anhydrous calcium sulfate. This product may be ground or comminuted to the desired degree of fineness and aged byexposure tothe air prior to use. Such some results in'the formation of th hemihydrate from the soluble anhydrite due to the reaction 01' the anhydrite with moisture contained in the air in accordance with the following equation:

2c+ cosh/z (3) The e p as indicated in the foregoing maniple z'except that lump or granular gypsum. is employed as a raw material, and the calcination is effected in the usual type of rotary calciner.

The products prepared in accordance with. the above indicatm methods are all closely similar in physical and chemical characteristics, and to all of them I apply the descriptive term normal pottery plasters." It is clear, however, that such normal pottery plasters may be prepared by a Heretofore there have been no means of n number of other methods which are well Mom in the art, and the term as used-herein may refer to any calcium sulfate hemihydrate which is produced by calcination at substantially at- "mospheric pressures and whichis treated to pro- 'duce a plaster which may be used in, making casting molds in accordance with-theprior' art methods. However, as indicated above, these prior art compositions are not entirely suitable for eastmold casting purposes. Examples of methods of producing normal pottery plaster" as herein defined are: (1) Ground gypsum rock is calcined inacalcining kettleinthepresence oia'small amount of a hygroscopic salt, such as calcium chloride, which may be added during or precedin; the calcination. The calcimn. chloride is added to produce a. Dre-aged low consistency product and the calcination is carried to the a 'point of producing calcium sulfate hemih'ydrate.

ing plasters because of the fact that when they are ground to a sufficient degree or fineness to form a smooth surface on the resulting cast; the normal consistency oi the product is raised bleyond the desired point for uses.

In the past, attempts have been made to prepare casting plasters by mixing normal pottery plaster as above defined with a type of calcium suliate hemihydrate which is known in the art as alpha gypsum. (Alpha gypsum may be prepared by calcining calcium sulfate dihydrate under superatmospherlc pressure as disclosed in United states Patents Nos. 1,901,051 and- 1,931,240 to Randel and Bailey. Alph ypsum is entirely diil'erent from ordinary alcined 87P- sum prepared by the usual calcining methods,

and is characterized by its normal low consistency and ability to form set products having very high strength.) Such attempts have ben unsatisfactory, because after a short period of use the molds have a tendency to develop rough finished surfaces. The alpha gypsum that has been used in these mixes in the past has contained a certain amount of coarse material and usually about 2 to 3 per cent of material which will not pass througha 100 mesh screen.

It is therefore an object of this invention toprovide a plastic composition that is adapted to wear uniformly without development of rough surfaces or high spots in the casting mold during tially entirely calcium sulfate hemihydrate which v when mixed with water produces a slurry substantially free from small bubbles of occluded air and which therefore can be used to produce molds whose surfaces will not, develop pin holes in use.

'Another object of this invention is to provide a type of calcium sulfate hemihydrate characterized by its ability to rehydrate'to a product which will resist recalcination.

A still further object of this invention is to provide a plastic composition particularly suitable for use as a pottery plaster capable of lowering mold costs and increasing economy in use by improved working.

i ,better uniformity,

mixing and aging properincreased life, and adaptability to a number of specific uses in the pottery industry.

Further and additional objects will appear from the following description and the appended claims.

Inaccordance with this'invention a casting plaster is prepared by adding to a normal pottery plaster a substantial amount of finely comminuted alpha gypsum, the alpha gypsum being comminuted to such an extent that it is substantially free from particles coarser than those:

which will just pass through a 100 mesh screen. I have discovered that the reason for the de-' velopment of rough mold surfaceson molds made from mixtures of alpha gypsum and pottery plaster is due to the differential rate of wear on such surfaces. The softer and weaker portions of the surfaces comprising rehydrated normal pottery plaster wear at a faster rate than the harder, denser'coarse particles of rehydrated alpha gypsum .which are surrounded. by the. relatively softer matrix of the former. Thus, aftera short period of use, the hard coarse particles of hydrated alpha gypsum project abovethe mold surface causing diameter (plus 100 mesh), are sufflciently large 'to cause the development of rough mold surfaces. However, satisfactory molds are obtained if sub -v stantially all of the particles greater than 150 microns average diameter (plus 100 mesh) are high spots" or mold roughness. This roughness is transferred to the surface of clay ware formed by removed, and. even better results are obtained if I substantially all of the particles having an av erage diameter of greater than '15 microns are removed. In any case, not more than 0.5% of the alpha gypsum should comprise particles greater than 150 microns average diameter; that is, less than 0.5% of the alpha gypsum should be retained by a mesh screen. Molds made from a mixture of normal pottery plaster and this finely comminuted alpha gypsum containing no particles coarser than the sizes indicated wear smoothly and-evenly in use without the development of rough surfaces, since there are no particles present of sufficient size to permit high spots to develop and project above the normal mold surface.

Normal pottery plaster now commonly used contains a considerable proportion of particulate material coarser than 100 mesh. Rough mold surface development caused by the presence of such'large particles in normal pottery plaster is not as pronounced as in a normal pottery plaster-alpha gypsum mix. Large particles in ordinary calcined gypsum (excluding impurities) are composed of agglomerates of myriads of exceedinglyfine calcium sulfate hemihydrate crystals.

The size of individual crystals making up these agglomerates does not exceed 2 to 5 microns in average diameter. When mixed with water, the aggregates break down into individual crystals, so that, when hydration occurs, a matrix of uniformly dispersed very fine gypsum crystals is formed. On the other hand, coarse particles of alpha gypsum are not loosely bound aggregates or agglomerates of individual crystals. Each particle is an individual crystal or crystal fragment in itself. When hydration of this product occurs, gypsum crystals grow from these fragments, the crystals being very dense at their center of formation adjacent the original hemihydrate crystal nucleus. For this reason, therefore, the presence of a small number of coarse particles in alpha gypsum is more deleterious than the presence of a similar number of like-sized coarse particles in the normally calcined pottery plaster.

As previously indicated, casting plasters have been prepared from norma1 pottery plaster. The plaster may be ground and aireseparated to such an extent that approximately 100 per cent of it will pass through a 100 mesh screen. However,

normal pottery plaster of this type, having no alpha gypsum incorporated therewith, is unsatisfactory for the following reasons: (1) Grinding or air-separation to produce the desired degree of fineness results in an increase in consistency of approximately 6 to 10 cubic centimeters in the finished product. This is undesirable because increased amounts of mixing water are necessary which results in set casts or molds having lower 40'and 45 cubic centimeters due to the crystalline,

nature of the alpha gypsum. It is readily soaked and mixed even when finely ground. A mixture of 30 per cent of finely ground alpha gypsum and 70 per cent of air-separated finely ground normal pottery plaster is an excellent casting plaster Plaster and -a mold surface. Due to this factandzalso due to the fact that my improved casting plaster possesses considerably greater strength and resistance to abrasion than present types of casting plaster, such as that prepared from normal pottery plaster, mold life may be increased up to 300 or 400 per cent. For certain types of pottery molds such as cup molds, cup handle molds, jigger molds, etc., heav- .ler and denser plaster molds than those which.

have been formerly obtainable are desirable. Such molds, due to their greater-density, are more resistant to efiects of abrasion and solution than are the lighter weight molds prepared in accordance with prior art methods. On the other hand, other types of pottery molds are sometime desired which'will possess a high rate of water absorption and a high total water absorption. These latter types of molds may be prepared from high consistency plaster. Specifically, molds of this latter type are employed in the so-called. slip casting process where a thin slurry of clay is poured within the plaster molds, the plaster acting to absorb excess water from the clay slip, whereby a thin film of solidified clay were is formed on the mold surface. Therefore, it is seen that it is desirable to have available in a pottery variable casting plasters of varying consistencies for varying types of molds. This invention renders possible the attainment of this "which socks and mixes easily to a smooth creamy. Table No. 1

mix, free from l mps and pin holes. Such a.

mix may be used to form molds which wear unimmm dumb-M formly in use without the production or develfli opment'of rough surfaces and high spots on the 5 Time oiexposum, hrs.

It is thus clear from the resists indicated in "l'ableqNo. 1 that a casting prepared by my imcalcined, reground and air-separated normal potdesideratum by varying the proportion of finely ground alpha gypsum which must be added to the finely ground normal pottery plaster to obtain a casting plaster having the desired consistency. By varying relative proportions of these two ingredients, I have found it possible to produce potteryplasters of any desired consistenc within the range of 44 to 70 cubic centimeters. Therefore, by following the teaching of this invention,

' any pottery can produce molds of maximum utility and life under all conditions which are encountered in the pottery.

The addition of finely ground'alpha gypsum in amounts in excess of per cent oithe normal pottery plaster present increases the resistance of the molds cast from such mixes to calcination under drying conditions which are encountered in potteries where drying of molds is desirable in order to permit rapid re-use thereof. By using a plaster which will produce molds that are more resistant to calcination under severe drying con+ ditions, more rapid le-use of molds is, possible.

.I have discovered that a mold prepared from finely ground alphagypsum and normal pottery plaster in accordance with this invention is considerably more resistant to calcination than'molds ,which are prepared inaccordance with the prior art processes from only normal pottery plaster without the addition of alpha gypsum. A casting mold prepared from 100 per cent normal pottery '70 per cent normal pottery plaster and per cent finely comminuted alpha gypsum were placed in a drier maintained at 150 F., 5 per cent relative humidity, feet per minute.

At difierentintervals of time the per cent of ypsum dehydrated in each mold was determined. 1 The results of these analyses amearinTableNa'l. Q r p I prefer to prepare it in accordance with the tery plaster, screening approximately 100 per cent through 100 mesh. The relative consistencies of the two ingredients separately-are about 40 to indicated above is approximately cubic centimeters, which is normal for certain types of casting plasters as has been indicated. By-increasing the proportion :of alpha gypsum,-the

consistency of the finished mix may be reduced to as low as 40 to 44 cubic centimeters, and by decreasing the proportion of alpha gypsum, the consistency of the finished plaster may be raised to approximately 62 to 67 cubic centimeters as desired.

To obtain alpha gypsumof the type described for use in the casting plaster-of this invention,

. methods disclosed in Randel and Dailey Patent No. 1,901,051 and Randel and Dailey Patent No. 1,931,240. The dried alpha gypsum product thus formed is then reduced to a fine-powder by any desirable combination of grinding methods, such asfor example. a hammer mill or attrition mill,

casting prepared from a mixture of having an air velocity of 300 preferably followed by ball milling or tube milling.

' The productat this stage normally'screens approximately to percent through a 325- meshscreen, but still contains 1 to 3 per cent which will not pass a'100 mesh screen, and 5 to '7 per cent which will not pass a 200 mesh screen. The material is then-passed through an air-separator which is adjusted to remove substantially all of the particles of alpha gypsum, coarser than those which will pass 200 mesh (greater than 75 microns average diameter). The finished product then screens approximately 100 per cent through 200 mesh and 90 to per cent through 325 mesh. of course-if desired. the entire mass of-material may be ground so fine that the desired degree of fineness is obtained without the use of. air-separation or mechanical screening. However, in commercial practice it has been found preferable to employ air-separationto remove the coarser p fl V the benefits of the present in- While most of vention may be obtained by employing.- a mix comprising less than 100 mesh size alpha gypsum .and normal pottery plaster, I have found it undesirable, because if they exceed a size greater than that which would pass through a 100 mesh screen, they tend to cause formation of high spots in the finished molds.

The improved casting plaster of this invention may be prepared directly at the manufacturing plant or the major ingredients, that is, the alpha gypsum and the normal pottery plastenmay be distributed separately to potteries or other users for preparation of the final mix in the relative proportions desired by the user. The constituents of the mix may be separately -ground, screened or air-separated prior to mixing, or the mixture may be so processed, if desired.

It is obvious that the qualities of the new plaster prepared in accordance with this invention make it suitable for other uses than for the preparation of pottery molds. For example, the mixture may be employed for molding plasters, gauging plasters, dental plasters, and the like. Mold life of cast stone molds is greatly increased by this improved product.

If desired, small amounts of other ingredients may be incorporated with the plaster of this invention to control such properties as setting time, setting expansion, mold surface hardness, and the like. Such ingredients may include ground raw gypsum, potassium sulfate, Portland cement, hydrated lime, commercial retarder, gum arabic, dextrin, boric acid, and the like.

While a particularembodiment of this invention is shown above, it will be understood, of course, that the invention is not to be limited thereto, since many modifications may be made, and it is contemplated, therefore, by the appended'claims, to cover any such modifications as fall within the true spirit and scope of this invention. I claim:

1. An improved casting plaster comprising a mixture of finely comminuted alpha gypsum, substantially all of the particles of which will pass through a 100 mesh screen, and finely comminuted normal pottery plaster.

- 2. An improved casting plaster comprising a 3. An improved casting plaster comprising mixture oi? finely comminuted alpha gypsum, substantially all of the particles of which have a size less than 150 microns average diameter, and finely comminuted calcium sulfate hemihydrate calcined under normal atmospheric pressure.

4. An improved casting plaster comprising a finely comminuted mixture of alpha gypsum and calcium sulfate hemihydrate calcined under normal atmospheric pressure, said mixture containing not over 0.5 per cent of material coarser than 100 mesh.

5.'The casting plaster recited in claim '3' will pass through a 100 mesh screen, and about '70 per cent by weight of calcium sulfate hemihydrate calcined under normal atmospheric pressure.

8. An improved casting plaster comprising a" mixture of about 30 per cent by weight of alpha gypsum and about per cent by weight of calcium sulfate hemihydrate calcined under normal atmospheric pressure, said mixture containing not ov r 0.5 per cent of particles having an avera e diameter greater than about microns. Y K a 9. An improved casting plaster comprising a mixture of alpha gypsum and calcium sulfate hemihydrate calcined under normal atmospheric pressure, said mixture comprising finely comminuted particles of a degree of fineness such that less than 0.5 per cent of the particles are coarser than 150 microns average diameter.

10. An improved plaster comprising a finely comminuted mixture of alpha gypsum and calcium sulfate hemihydrate calcined under normal atmospheric pressure, said mixture consisting of particles of such size that less than 0.5 .per cent of them are coarser than about150 microns average diameter, and said mixture containing a sufilcient proportion of alpha gypsum to lower the normal consistency thereof toless than 60 cubic centimeters.

11. A process of producing an improved casting plaster comprising mixing alpha gypsum and calcium sulfate-hemihydrate calcined at normal atmospheric .pressure, finely comminuting the mixture, air-separating the resulting mass,

and separately collecting asthe desired product that portion of the mass consisting substantially entirely of particles having an average diameter of less than about 150 microns.

c. BAILEY.- 

